赤・赤い

Aka or akai: which one do I use?

Both words mean red. 赤 (aka) names the colour. 赤い (akai) describes something that is red, such as an apple or a pen.

The colour name (noun)

赤 (あか) · aka

Use this when you are talking about red as a colour, such as saying that you like red.

The describing word (い-adjective)

赤い (あかい) · akai

Use this to describe a red thing. Put it directly before the thing: 赤いペン means “a red pen.”

To say that something is red, you can also use 赤いです at the end of a polite sentence. The final い belongs to the adjective; keep it in 赤いペン and 赤いです.